Competition from materials other than titanium for the use in military and other aircraft (the chief use of alloyed titanium metal) has caused a re-evaluation of the method of preparing the metal for use. While titanium has the best strength-to-weight ratio of any metal, the fact that several pounds of titanium are required to put one pound in place on the aircraft has made the use of titanium prohibitively expensive. Also scrap titanium is difficult to identify, segregate and re-use.
One solution to the high cost of titanium has been a process known as hot isostatic pressing. Here, titanium alloy powder is formed to approximate but oversized dimensions by pressing at high temperatures. Good powder can be pressed to 100 percent of theoretical density. Considerably less scrap results, since only that material which causes the object to be oversized need be removed to bring the piece to the desired dimensions.
Unfortunately for the industry, no consistent, cheap process now exists for producing a specification alloy powder. Apparently, regardless of the process used, the presence of chloride hinders the making of coherent metal.
In addition, the industry has long been plagued by too much oxygen and nitrogen, which are introduced in the grinding, washing and drying steps (see Bureau of Mines report of investigations 4519, August 1949). No washing and drying are required in the practice of U.S. Pat. Nos. 3,801,307 and 4,032,328 by the applicant herein; hence no oxygen, nitrogen or even hydrogen is introduced by these processes. Even the removal of chloride by vacuum distillation has left the titanium "too dirty". (See USBM RI 4837, February 1952). Tests of the prccess of U.S. Pat. No. 3,801,307 have shown very low chloride levels in the resultant metal.
Titanium alloy rather than pure titanium is sought to be employed in aircraft manufacturing. The problems of forming titanium alloys of high purity which are uniform in composition and characteristics is difficult and frequently tends to exacerbate the problems discussed herein above. As discussed above, one of the most promising approaches has been consolidating oversized, highly compressed parts of titanium powder by heating under pressure or vacuum. It has now been possible to achieve over 99 percent theoretical density and to arrive at strength in the fatigue characteristics comparable to that of wrought metal. Some characteristics such as machinability are superior. Two types of powder compacts are under development. Pre-alloyed powder, wherein each powder particle is a uniform alloy and elemental blend, and where pure titanium powder is intimately mixed with powdered alloying metals. The former has been successfully fabricated using hot isostatic pressing, while the most successful elemental blend compacts have been processed under vacuum at temperature. In either case, the consolidation temperature has been above that where alpha phase titanium transforms into beta phase. The pre-alloyed powder is generally considered to be superior in physical properties to the elemental blend powder, particularly in the area of fatigue resistance. A drawback with the elemental blend powder is said to be the high chloride content of the elemental titanium.
The pre-alloyed powder is prepared by disintegrating solid titanium metal which has been fully processed through the stages of sponge manufacture, alloy blending, compaction and consolidation into solid material by consumable electrode arc melting, and conversion to a form suitable for reduction to powder. Since this powder has been arc-melted, all chloride has been volatilized out of it. Also, each resultant powder particle is quite uniform in alloy content.
The elemental blend powder contains the chloride of its parent material and incomplete alloying may be present because the only way the alloying elements can be mixed with titanium is by diffusion. Some degree of in-homogeneity may be anticipated.